Inorganic cellular phosphate (Pi) has been difficult to measure accurately in the renal proximal tubule due to methodologic inaccuracy and high rates of Pi transport. A perfusion 31P-NMR probe has been specifically designed to address the inter-relationship between modulators of Pi transport and metabolism and free cellular Pi in these cells. Sensitivity of the perfusion 31P probe enables minute-to-minute monitoring of the cellular pool of Pi with extra-and intra-cellular modulators of metabolism and transport.